Glass material with darker areas Cycles
I've recreated your scene, and after examining the renders, it seems that the HDRI/Scene that you've created for lighting is causing the dark spots;



Try using another HDRI, or rotating the view, to see what I mean. I'm using this HDRI;



Which can be found in various resolutions here;

https://hdrihaven.com/hdri/?c=indoor&amp;h=lebombo

Edit:

I see. Forgive me, I don't eat gnutella, and so our bottles were totally different. While mine was perfectly round, yours has a very unique (and by the way nicely modeled) shape;



Your specific issue is the table (you didn't pack the images, so this is going to look a bit odd);





If I change the table to a darker color, you see that the issue with your bottle isn't dark parts, it's an especially light table, contrasting against a dark background;



I bet you could replicate these results in real life with a bottle of gnutella and a table.